The desire for semiconductor chips with memory structures denser than 256 megabytes has created the demand for machines and techniques which allow the etching of ultrafine lines of impurities in semiconductors. Some of the techniques currently used to etch such ultrafine lines are X-ray, electron beam, and ion beam lithography.
Of the aforementioned techniques, the X-ray method is believed to be the most advanced. Compact synchrotrons are being tested and improved as X-ray beam sources for use in chip making. However, X-ray lithography only has a feasible resolution of about 0.2 microns.
In order to overcome this resolution barrier, an approach using particles, such as atoms, electrons, or ions, is preferred to an approach using X-rays for several reasons. First, particles do not penetrate as far below a semiconductor substrate surface as X-ray photons. This results in the ability to etch relatively finer lines on a target. Second, shorter wavelengths, which also enable the etching of finer lines, can be more easily provided for particles than for X-ray photons. In this regard, the energy required to produce a particle beam of a given wavelength is far less than that required to produce an X-ray photon beam of the same wavelength and intensity.
The ion beam lithographic method is preferable over both the X-ray and the electron beam method because the relatively heavier ion particles couple well with various resist materials, do not damage an underlying circuit layer, scatter weakly, and allow for image demagnification.
However, there are at least two significant problems with the ion beam approach. One problem is that, in order to treat solid state surfaces at a high rate, an ion beam of high enough current density must be used. At high current densities, electrostatic repulsion of ions strongly distorts the image or the pattern etched. A second problem is that existing ion sources cause high ion temperatures which can result in the thermal distortion of the etched image. Therefore, in order to produce a heavy particle beam approach to lithography which is to be competitive or better than current X-ray approaches, one has to first overcome the problems of electrostatic and thermal distortion of the etched image while producing a sufficiently intense particle beam.